The present invention relates to a hologram disk for deflecting a light beam.
Hologram scanners are known for use in optical scanning mechanisms such as in optical printers or the like. The hologram scanner includes a hologram disk rotatable about its own axis by a motor.
The hologram disk comprises a circular plate on which a plurality of mutually equivalent straight-line gratings are formed by holograms as an annular pattern along the outer peripheral edge thereof. A diffracted light beam is produced by applying a light beam in a certain direction to a straight grating. By rotating the hologram disk with the motor, the produced diffracted light beam can be deflected periodically. The diffracted light beam is deflected as the direction of grating lines with respect to the incident light beam is varied upon rotation of the hologram disk. The diffracted light beam is periodically deflected since the incident light beam falls upon different straight-line gratings successively as the hologram disk rotates.
Heretorefore, the straight-line gratings used in the art comprise equidistant-straight-line gratings having a series of fine straight lines that are closely spaced at equal intervals.
For good optical scanning, a diffracted light beam should preferably be deflected by a hologram scanner at a uniform angular velocity (hereinafter referred to simply as a "uniform velocity"). The conventional hologram disks include equidistant-straight-line gratings produced by holograms. This design has failed to deflect the diffracted light beam exactly at a uniform velocity. For example, FIG. 6 of the accompanying drawings illustrates the relationship between angles through which a diffracted light beam is deflected and errors of the uniform velocity at which the diffracted light beam is deflected for scanning, when .lambda./d=1.4 (where .lambda. is the wavelength of the incident light beam and d is the spacing between adjacent grating lines) and the angle .theta.i of incidence of the incident light beam (the angle between a line normal to the grating surface and the incident light beam) is 44.25.degree.. In the illustrated example, the uniform velocity error when the maximum beam deflecting angle is .+-.21.degree. reaches +1.16%.